In the field of television, considerable efforts have been directed toward digitizing the color video signal, processing the digitized samples of the video signal (a) to separate the chrominance and luminance components and (b) to demodulate the chrominance components into respective baseband signals, and then converting the digital samples back into respective analog signals for the application thereof to the television picture tube for reproduction. A motivation for these efforts comes from the fact that the digital television can offer a number of novel features--such as still picture display, multipicture displays, direct hookups to satellite dish amplifiers, etc. The digitization is typically achieved by sampling the analog video signal at a finite sampling rate, which must exceed a predetermined minimum sampling rate in order to keep the quality of reproduction within acceptable limits.
The minimum sampling rate must satisfy what is generally known as the Nyquist criterion, which requires that the sampling rate be at least twice the bandwidth of the analog signal of interest. In the NTSC format of the color television system, the desirable signal bandwidth is about 4.2 MHz, thereby requiring a sampling rate in excess of 8.4 MHz. If the sampling rate is higher than the minimum value given by the Nyquist criterion, then aliasing of the digital samples is avoided.
Because of the operational considerations, it is advantageous to sample the analog color video signal at some integral multiple of the frequency of the unmodulated color subcarrier, hereinafter referred to as Fsc (3.58 MHz). The sampling rate of 3 Fsc is the lowest integer multiple of the color subcarrier frequency that exceeds the Nyquist requirement. However, the 3 Fsc sampling rate poses certain operational disadvantages in the signal processing operations of the television receiver--such as the demodulation of the chrominance components into the respective baseband signals. It is, therefore, common practice to use a sampling rate that is four times the color subcarrier frequency (4 Fsc), although it results in a far greater sampling rate than is called for by the Nyquist criterion.
After the incoming color video signal is digitized and decoded into its respective baseband components--i.e., one luminance (Y) and two color difference signals (I and Q, for example), it may be desirable to store the digital samples in a field or frame memory for reasons such as progressive scanning, noise reduction, special effects, etc. At this stage, it is possible to reduce the size of the memory by reducing the sample rate of the stored data from 4 Fsc to something lower without violating the Nyquist criterion.
In accordance with this invention, the sample rate of the luminance signal is reduced from 4 Fsc (i.e., 14.32 MHz) to (8/3) Fsc (i.e., 9.55 MHz). The choice of two-thirds as the multiplier not only facilitates the sample rate reduction process, but is also fulfills the Nyquist requirement that the sample rate (8/3 Fsc or 9.55 MHz) exceed two-times the highest signal frequency (8.2 MHz) in the luminance band.
The sample rate reduction for each of the chrominance components (e.g., I and Q) may be greater (e.g., one-third or one-fourth or less of the original sample rate of 4 Fsc), since the desired bandwidths for the chrominance signals are much lower compared to the luminance signal (e.g., 1.5 and 0.5 MHz, respectively). To this end, suitable sample dropping or decimating circuits may be employed for reducing the sample rates of the chrominance components. The specific sample reduction circuits for the chrominance signals are not a part of this invention.
The sample rate reduction apparatus, pursuant to this invention, receives the input sample stream and generates an interleaved output sample stream in which half of the output samples are passed unaltered from the input sample stream and in which the other half of the output samples are interpolated from the original input samples, and which has a sample rate that is two-thirds of the input sample rate.
In one embodiment of this invention, the sample rate reduction apparatus includes a set of three latches connected together in series. The input sample stream is clocked through these latches in response to the accompanying 4 Fsc clock pulses, thereby sequentially making available to an interpolator successive sets of four input samples. The interpolator generates at the output thereof a stream of interpolated samples in response to the (8/6) Fsc clock pulses. A switch is alternately coupled to the interpolator output and the output of the second one of the three latches at the (8/6) Fsc rate to merge the streams of the interpolated and unaltered input samples. A fourth latch connected to the output of the switch and driven by the (8/3) Fsc clock pulses provides an output sample stream which has a sample rate that is two-thirds of the sample rate of the input sample stream, and in which one-half of the output samples are interpolated and the other half are unaltered input samples.
An advantage of the subject apparatus for generating an interleaved output stream is that the coefficients employed for estimating the interpolated samples do not change from cycle to cycle, since the relative position or timing of the interpolated samples remain fixed relative to the respective input samples.